I Is causal structure in classical GR still an open area of research?

I am currently taking graduate course in GR following Carroll and Wald and we just started covering causal structure. In all my years studying physics this has been the first time something made me stop and go "I would love to spend a decade on this". To me it's a perfect blend of well posed mathematical problems and fundamental questions about reality. However I don't know if this is still an active area of research. If anyone can shed any light on this I would really appreciate it.